1. Field of the Invention
The present invention relates to a technique for measuring spectral reflectance of a measurement object and a technique for measuring a thickness of a film (or thicknesses of films) formed on a measurement object.
2. Description of the Background Art
In order to ensure consistent quality of semiconductor products, it is usual to measure thicknesses of films (e.g., layered thin films) formed on a semiconductor substrate (hereinafter, referred to as “substrate”) in an inspection step of a semiconductor manufacturing process. For measurement of film thicknesses, a film thickness measurement apparatus has been conventionally used, adopting a light interferometric method where film thicknesses are calculated on the basis of spectral reflectance obtained by emitting white light to a substrate and performing spectrometric analysis of the light reflected on the substrate.
Japanese Patent Application Laid Open Gazette No. 10-142053 (Document 1) discloses a technique for obtaining spectral reflectance where an image of a sample in which a white area is surrounded by a black area is picked up by irradiating the sample with white light, to obtain a flare spread function on flare light which affects pixels corresponding to the black area on the sample image from the white area and then an influence of the flare light on a color patch from other color patches in an image of a color chart is removed by using the flare spread function.
In recent years, however, another technique has been attempted, for obtaining spectral reflectance on the basis of a plurality of single-band images (so-called, “a multiband image”) corresponding to a plurality of wavelengths, which are acquired by sequentially emitting lights of a plurality of different wavelengths to a substrate. Since a pixel value in a single-band image which corresponds to a predetermined measurement point on a substrate is affected by wraparound of light from its surrounding area (in other words, stray light in the S-V effect), however, it is necessary to block the light in an unnecessary area with a member having a pinhole in order to measure spectral reflectance with high accuracy, and therefore it is impossible to obtain a reflectance on the basis of an image with accuracy. According to “Spectroscopy Handbook” edited by Shigeo Minami and Youichi Goushi (published by Asakura Shoten, 1990, pp. 213–214), the S-V effect refers to a phenomenon where the apparent photometric quantity increases due to the flare light caused by an optical system and the like in a measurement apparatus to cause some photometric errors.
The technique of Document 1, where the spectral reflectance of each color patch is measured, assuming that the influence of the flare light caused by a while area in a sample on pixels corresponding to a black area surrounding the white area and the influence of the flare light caused by a color patch in a color chart on pixels corresponding to other color patches in an image are equated with each other, also disadvantageously causes degradation in accuracy of measurement.